(BARCELONA, Spain) - Recent research indicates a new hormone-blocking drug works better than the standard medicine in preventing women with early-stage breast cancer from developing tumors in the healthy breast.
In the largest breast cancer treatment study to date, women taking Arimidex were less than half as likely as those taking tamoxifen to develop a new cancer in the other breast.
The study, paid for by Arimidex's maker, AstraZeneca, was presented Thursday at the European Breast Cancer Conference in Barcelona, Spain.
International experts were enthusiastic about the findings, and some said it provided a clue that the drug could be a better option than tamoxifen for preventing breast cancer in healthy women.
However, others expressed concern that the women taking Arimidex had more bone fractures than those taking tamoxifen, an indication the new drug accelerates bone loss.
About 700,000 new cases of breast cancer are diagnosed every year worldwide. Estrogen fuels the growth of more than half of all breast cancers, especially those in older women. Tamoxifen, the top hormonal treatment for estrogen-fueled tumors, prevents estrogen from linking up to a receptor on the surface of cancer cells.
Arimidex is among a new class of cancer drugs called aromatase inhibitors, which work differently _ by blocking production of estrogen. They are used mostly when tamoxifen fails in advanced breast cancer.
They do not work so well on cancers that are not driven by estrogen, nor on pre-menopausal women because the drug becomes overwhelmed by the amount of estrogen that women of childbearing age produce.
``We know that women successfully treated for early breast cancer still have a threefold increased risk of developing a new tumor in the opposite breast compared with women who have not had breast cancer,'' said the study's leader, Dr. Jeffrey Tobias, a breast cancer specialist at University College Hospital in London. ``Tamoxifen reduces this risk by nearly a half. To find a treatment that cuts this risk in half again is truly remarkable.''
The study involved 9,366 post-menopausal women with early stage breast cancer followed so far for nearly three years. One-third of them were given tamoxifen after their tumors were removed, another third got Arimidex and the rest got both the drugs.
So far, 14 women, or 0.4 percent, in the Arimidex group have developed a new tumor in their other breast, compared with 33 women, or 1.1 percent, in the tamoxifen group. That is a difference of 58 percent.
When the scientists restricted their calculations to the women whose cancers they knew were driven by estrogen _ 84 percent of the total sample _ the risk of new cancers in the opposite breast with Arimidex was 64 percent lower than with tamoxifen.
Giving a combination of tamoxifen and Arimidex was no better than tamoxifen alone, and no safer.
The women on Arimidex had fewer endometrial cancers, were half as likely to have vaginal bleeding and half as likely to develop dangerous blood clots _ the most worrying side effect of tamoxifen.
Dr. Alan Coates, chief executive of The Cancer Council for Australia, said the favorable safety evidence for Arimidex makes it an appealing candidate for breast cancer prevention.
``If women are at risk of dying from breast cancer, you can tolerate some side effects. But if you are going to treat thousands of well women in the hope of preventing a handful of cancers you really can't afford many side effects at all,'' said Coates, who was not connected with the research.
However, the Arimidex group had more bone fractures than the tamoxifen group _ 180 compared with 113. Experts say that is because the drug virtually eliminates estrogen in post-menopausal women.
``Putting women in a situation where they've got no estrogen will just lead to skeletal collapse. Ten years of that is really going to be seriously damaging,'' said prominent breast cancer scientist Sir Richard Peto, a professor of epidemiology at Oxford University who was not involved in the study. ``It's pretty squalid, so you can't really go long-term on something that's going to involve that.''
However, Coates and Peto said it may be possible to overcome that problem by giving non-estrogen bone-building supplements.
``Anything that's better than tamoxifen, which has been the best drug for a quarter of a century, has to be taken seriously,'' Coates said. ``This is a very big, very reliable trial showing that we can do better in terms of reducing recurrences and reducing some of the side effects. This suggests it might be the preferred drug for prevention, so long as we can overcome the bone worries.''
On the Net: European Breast Cancer Conference